1. Field of the Invention
The present invention relates to a transfer assembly to transfer an image from an image carrying member to a transfer member such as a recording medium at a transfer nip, set between the image carrying member and a recording medium, and an image forming apparatus using the transfer assembly.
2. Description of the Background Art
Typically, an image forming apparatus includes an image carrying member and a counter member opposing the image carrying member. The image carrying member and counter member form a transfer nip therebetween, at which an image can be transferred from the image carrying member to a recording medium such as a sheet of paper, etc. Specifically, the counter member may be pressed toward the image carrying member using a force of a pressure device to contact the image carrying member to form the transfer nip. In addition, the counter member can be separated from the image carrying member using a separation device as required. When a recording medium is a thick sheet such as a thick paper, shock jitter may occur at the transfer nip, and banding (i.e., uneven image concentration appearing as lines on an image) may occur. Such banding phenomenon may occur when the thick paper enters the transfer nip, because the image carrying member may receive a greater load abruptly or within a short time, and as a result the image carrying member experiences a moment of steep drop in line speed.
JP-H10-83124-A discloses a method of preventing shock jitter, in which a transfer roller is used as the counter member. The transfer roller includes a cylindrical roll and a shaft projecting from both end of the roll, and the roll and shaft rotate integrally. Further, a rotatable cam disposed at each end of the shaft can rotate freely at each end of the shaft without force transmission between the cam and the shaft.
The rotatable cam, which can rotate freely on an outer face of the shaft, has a convex portion at a given rotation angle position abut-able against an axial end portion of an image carrying member such as a photoconductor. With such abutting action, the transfer roller, pressed toward the photoconductor by a pressure device, can be forcibly moved away from the photoconductor against the force, by which a shaft-to-shaft distance between the photoconductor and transfer roller can be adjusted. For example, when thick paper is used as the recording medium, the transfer roller can be forcibly moved away from the photoconductor by the rotatable cam to decrease a transfer pressure by enlarging the shaft-to-shaft distance (i.e., separating the transfer roller from the photoconductor). With such a configuration, an abrupt load increase at the photoconductor, which may occur when thick paper enters the transfer nip, can be suppressed or prevented. However, although an abrupt load increase to the photoconductor can be prevented or suppressed by setting the shaft-to-shaft distance wider, as a side effect an effective transfer pressure may not be set, and thereby transfer failure may occur.
JP-H06-274051-A discloses an image forming apparatus in which a transfer roller can be separated from a photoconductor by driving a rotatable cam by activating a solenoid before a thick sheet of paper, used as a recording medium, enters a transfer nip, in which a minute gap may be set between the transfer roller and photoconductor to prevent the occurrence of shock jitter. Then, right after the front edge of the thick paper enters the minute gap, the solenoid is deactivated to cancel a forced separation of transfer roller so that the transfer roller can be pressed toward the photoconductor by a force of a spring used as a pressure device. With such a configuration, the transfer roller is separated from the image carrying member until a recording medium such as a thick sheet of paper enters the transfer nip, and thereby a load increase at the image carrying member when the recording medium enters the transfer nip can be suppressed. However, when separation of the transfer roller is canceled, the image carrying member, the recording medium, and the transfer roller may instantly collide with each other due to the force of the pressure device, thereby causing a load increase or vibration at the image carrying member with possible image failure (or image deterioration) as a result.